1. Field of the Invention
The present invention relates to a both-face (i.e. duplex) image forming apparatus, such as a copier, a printer or a combination of the two, in which a front surface and a back surface of the sheet are reversed, and images are formed on both surfaces of the sheet. As used herein, duplex and both-face are used interchangeably.
2. Related Background Art
A conventional example of a both-face image forming apparatus is a copier.
The image forming processing performed by a copier 17 will be described while referring to FIG. 7. A rotatably supported image bearing member (hereinafter referred to as a photosensitive drum) 1 is rotated in the direction indicated by an arrow, while the surface is uniformly charged by a primary charger 2. Then, an image information exposure 3 is performed for the photosensitive drum 1, and an electrostatic latent image is formed on its surface. Thereafter, a developing device 4 performs a visualization process for the electrostatic latent image and produces a toner image.
Synchronized with the rotation of the photosensitive drum 1, a sheet P, which is a recording medium, is fed to the photosensitive drum 1 by registration rollers 11, and the toner image on the photosensitive drum 1 is transferred to the sheet P by a transfer charger 5. Then, the sheet P is separated from the photosensitive drum 1 by a separation charger 6. Following this, the sheet P is conveyed by a conveying unit 7 to a fixing device 8, whereat the toner image is fixed to the sheet P.
After the toner image has been transferred, the surface of the photosensitive drum 1 is cleaned by a cleaner 9, and the potential held by the photosensitive drum 1 is eliminated by a pre-exposure lamp 10. In this fashion, the photosensitive drum 1 is again prepared for the forming of another image.
The photosensitive drum 1, the primary charger 2, the image information exposure 3, the developing device 4, the transfer charger 5 and the separation charger 6, together constitute a so-called Carlson process type image forming means 16 for the copier 17, which is used to transfer a toner image from the photosensitive drum 1 to a plain paper sheet P.
For the image forming means 16, the transfer charger 5 applies, from the side opposite to the transfer surface of the sheet P contacting the photosensitive drum 1, an electrical field that has a polarity opposite to that of the charge polarity held by the toner, and thus induces the transfer of the toner image from the photosensitive drum 1 to the sheet P. Immediately after the transfer of the toner image to the sheet P by the transfer charger 5, the separation charger 6 separates the sheet P from the photosensitive drum 1. For this separation process, to eliminate the electrical charge held by the sheet P and to eliminate the attractive force existing between the sheet P and the photosensitive drum 1, the separation charger 6 applies to the sheet P an AC discharge or a DC discharge which has the same polarity as the toner. Then, the rigidity of the sheet P or its weight is employed to separate the sheet P from the photosensitive drum 1.
For a smooth separation, it is preferable that the sheet P be curled, for example, in an effective separation direction.
The effective separation direction is the direction in which, as is specifically shown in FIG. 10A, the leading end of the sheet P progresses while directed toward the separation charger 6 (away from the photosensitive drum 1) after passing through the photosensitive drum 1. The ineffective separation direction is the direction in which, as is specifically shown in FIG. 10B, reversely, the sheet P progresses while being curled around the photosensitive drum 1 and adheres to the surface of the photosensitive drum 1.
Next, the sheet conveying operation of a conventional copier 17 will be described while referring to FIG. 8.
Individual sheets P are shown in FIG. 8 so that it can be easily understood in which direction a sheet is curled when it is fed along the sheet conveying path.
Sheets P, stacked in a sheet feed cassette 14e and in sheet feed decks 14a, 14b, 14c and 14d, are fed to the registration rollers 11 through sheet feeding rollers 13, positioned on sheet feeding paths 50 and 51. Before a sheet P is fed to the registration rollers 11, in order to facilitate its separation from the photosensitive drum 1 it is curled a predetermined curling distance in the effective separation direction by an initial curling unit 12 or 20. By the aid of the curl, the sheet P will smoothly separate from the photosensitive drum 1. The initial curling units 12 and 20 individually comprise curling roller sets 12a and 12b, and 20a and 20b, each set of which is respectively composed of rollers having a small diameter and a large diameter.
As concerns the forming of images on both surfaces of a sheet P, a sheet P, to one surface (also called a first surface for this invention) of which an image has been fixed by the fixing device 8, is conveyed to a switchback type surface reverse portion (surface reversing means) by a flapper 15. The sheet P is routed by the surface reversing means so that it is reversed, and an image is formed on its other surface (also called a second surface for this invention). Thereafter, the sheet P is discharged from the copier 17.
The sheet P can be a sheet of plain paper, a sheet of thin resin that is substituted for plain paper, a post card, a sheet of cardboard, an envelope, or a sheet of thin plastic.
When a conventional copier 17 is employed to form images on second surfaces, however, the sheets P do not separate smoothly from the photosensitive drum 1, and there are frequent paper jams. Such jamming occurs because the sheets P are curled in the ineffective separation direction before images are formed on their second surfaces.
Assume that the measurement of the curling distance applied by the initial curing unit 12 or 20 is A (mm), and the measurement of the curling distance applied by a first R portion 18a and a second R portion 18b of the surface reverse portion 18 is B (mm).
It is expected that a sheet P will not be curled by components other than the initial curing unit 12 or 20 until it reaches the surface reverse portion 18, and that while passing through the surface reverse portion 18, the sheet P will be cooled and curled. However, while the sheet P will be curled by the first R portion 18a, it will seldom be curled by the second R portion 18b. 
The measurement of the curling distance in this invention indicates the degree to which a sheet if curled, and is a value obtained by performing a measurement such as is shown in FIGS. 9A and 9B. Specifically, as is shown in FIG. 9A, the curled sheet P is suspended, its curled edge Pa held horizontally, and the shape formed by the curled sheet is written on a horizontal plate 19, as is shown in FIG. 9B. Then the distance L, between a line D connecting the upstream end Pb and the downstream end Pc in the sheet conveying direction and the surface of the curled portion farthest from the line D, is measured, and the obtained value is used as the curling distance.
The curling distance for the first surface is the curling distance A (mm) in the effective separation direction. The curling applied to the first surface acts with the curling applied by the first R portion 18a to the second surface in the ineffective separation direction, and only the initial curling unit 20 acts in the effective separation direction. Thus, the curling distance is xe2x88x92Axe2x88x92B+A=xe2x88x92B (mm).
As is described above, with a conventional copier, while the first surface of a sheet can be satisfactorily separated from the photosensitive drum 1, the direction of the curling of the first surface applied by the initial curling unit 12 or 20 is opposite to the direction of the curling applied to the second surface by the initial curling unit 20. As a result, the effect provided by the curling rollers in the initial curling unit 12 or 20 is not offset for the second surface, and only the curling applied by the first R portion 18a in the ineffective separation direction remains.
Therefore, for a conventional image forming apparatus, separating a sheet from the photosensitive drum is difficult, and paper jams occur easily.
Furthermore, since as is shown in FIG. 11 the rollers 12a and 20a, which have small diameters, are longer than the large diameter rollers 12b and 20b and are apt to slightly bent, the pressure with which the ends of the roller 12b, or 20b, are pressed against a sheet may be greater than the pressure with which the intermediate portion of the roller 12b, or 20b, is pressed against the sheet.
Therefore, both ends of the sheet in the widthwise direction of the sheet are excessively curled, which prevents the sheet from being uniformly attached to the photosensitive drum 1. Accordingly, during the transfer process an aberration of the sheet may occur, or an image formed on the sheet may be blurred.
It is, therefore, an object of the present invention to provide a both-face image forming apparatus wherein, during the image forming process, the first surface of a sheet is curled in the effective separation direction to ensure its satisfactory separation from a photosensitive member, and in addition, the second surface of the sheet is also curled in the effective separation direction to ensure its satisfactory separation.
It is another object of the present invention to provide a both-face image forming apparatus wherein an aberration of a sheet and the blurring of images formed on sheets are prevented by suppressing the tendency to produce excessive curling at the ends of sheets.
To achieve the above objects, according to the present invention, an image forming apparatus comprises:
sheet stacking means for stacking sheets;
image forming means for forming an image, by a photosensitive member, on a sheet that is guided and fed along a sheet feeding path from the sheet stacking means;
initial curling means, positioned on the sheet feeding path, for curling the sheet in a direction away from the photosensitive member when the sheet passes through the photosensitive member;
a sheet conveying path along which the sheet, on one surface of which an image has been formed by the image forming means, is guided to the sheet feeding path;
surface reversing means, positioned on the sheet conveying path, for reversing a front surface and a back surface of the sheet; and
recurling means, positioned on the sheet conveying path, for curling the widthwise ends of the sheet in a direction away from the photosensitive member when the sheet delivered along the sheet conveying path passes through the photosensitive member.
In the both-face image forming apparatus, a sheet is fed along the sheet feeding path to the image forming means, and an image is formed on one surface of the sheet. During this feed of the sheet, the sheet is initially curled by the initial curling means in the direction that facilitates the separation of the sheet from the photosensitive member after the sheet has been past it, i.e., the direction in which the upstream end and the downstream end of the sheet in the sheet feeding direction are separated from the photosensitive member. That is, the sheet is curled in the effective separation direction.
Therefore, after an image has been formed on one surface, the sheet can be smoothly removed from the photosensitive member.
However, at this time the widthwise ends of the sheet, which were curled by the initial curling means, are curled toward the photosensitive member, and thereby, closely attaching the widthwise intermediate portion of the sheet to the photosensitive member would be difficult.
In the both-face image forming apparatus, after an image has been formed on one surface of a sheet, the surfaces of the sheet are reversed by the surface reversing means. But by reversing the sheet, the reversing means sets it so it is curled in the ineffective separation direction. Therefore, while the sheet is passing along the delivery route extending from the reversing means to the image forming means, the recurling means curls the sheet so it is again curled in the effective separation direction. At the same time, the curls at the widthwise ends of the sheet are removed and the ends are flattened, so that the image forming means can form an image on the other surface of the sheet.
The recurling means may also be positioned on the sheet conveying path upstream of the reversing means. In this case, when the sheet reaches the reversing means it has already been recurled, and thus, at the reversing means it is curled in the direction that is the opposite of that in which it was curled by the recurling means. However, since some of the curl applied by the recurling means is retained by the sheet, it is not difficult to separate it from the photosensitive member after an image has been formed on the second surface.
Therefore, compared with the conventional apparatus, the both-face image forming apparatus of this invention employs the recurling means for recurling the sheet in the effective separation direction, and to flatten it in the widthwise direction. Thus, after the image forming means has formed an image on the other surface of the sheet, the sheet can more easily be separated from the photosensitive member than can a sheet in the conventional art.
For the both-face image forming apparatus of this invention, the recurling means may be located between the initial curling means, which is positioned on the sheet feeding path leading from the sheet stacking means that is nearest the surface reversing means, and the sheet stacking means.
For the both-face image forming apparatus, since a sheet fed from the sheet stacking means nearest the surface reversing means is curled by both the recurling means and the initial curing means before an image is formed on one surface of the sheet, the sheet can easily be separated from the photosensitive member.
For the both-face image forming apparatus of this invention, the recurling means may be located between the surface reversing means and the sheet stacking means nearest the surface reversing means.
For the both-face image forming apparatus, since the recurling means is located between the surface reversing means and the sheet stacking means nearest the surface reversing means, the same curling measurement can be provided for all the sheets that are recurled.
For the both-face image forming apparatus, the initial curling means and the recurling means have a roller having a large diameter and a roller having a small diameter for holding and curling a sheet. The length of the roller of the recurling means that has a large diameter is shorter than the width of the sheet, and is greater than the length of the roller of the initial curling means that has a large diameter.
In the both-face image forming apparatus, the recurling means curls the sheet to remove the curl applied by the initial curling means at the widthwise ends of the sheet.
For the both-face image forming apparatus of this invention, the roller having a large diameter may be more elastic than the roller having a small diameter.
For the both-face image forming apparatus of this invention, provided at both ends of the roller having a large diameter are holding rollers that contact the roller having a small diameter when the roller having a large diameter is elastically deformed by the roller having a small diameter. The total length of the roller having a large diameter and the holding rollers, and the length of the roller having a small diameter may be set equal to or greater than the width of the sheet.
In the both-face image forming apparatus, when the roller having a large diameter is elastically deformed by the roller having a small diameter, and after in the feeding direction the upstream end and the downstream end of the sheet are curled, the roller having a small diameter feeds the sheet by contacting the holding rollers via the widthwise ends of the sheet.
For the both-face image forming apparatus, the roller having a small diameter is made of a hard metal, and the roller having a large diameter is made of sponge.
In the both-face image forming apparatus, the sheet is curled by pushing a part of the roller having a small diameter into the roller having a large diameter.
For the both-face image forming apparatus of this invention, the depth to which the roller of the recurling means having a small diameter bites into the roller having a large diameter is greater than the depth to which the roller of the initial curling means having a small diameter bites into the roller having a large diameter.
For the both-face image forming apparatus of this invention, the force with which the roller having a small diameter of the recurling means presses against the roller having a large diameter is greater than the force with which the roller having a small diameter of the initial curling means presses against the roller having a large diameter.
For the both-face image forming apparatus of the present invention, the roller of the recurling means having a large diameter may be more elastic than the roller of the initial curling means having a large diameter.
For the both-face image forming apparatus of the present invention, the diameter of the roller of the recurling means having a small diameter may be smaller than the diameter of the roller of the initial curing means having a small diameter.
For the both-face image forming apparatus, when the recurling means curls a sheet, it provides a greater curling distance than does the initial curling means. Thus, the sheet that has been curled by the surface reversing means in the direction in which it can be ineffectively separated from the photosensitive member is now curled in the direction in which it can be easily separated.
For the both-face image forming apparatus, the initial curling means and the recurling means may each include a pair of pressing rollers separately positioned in the sheet feeding direction, and one pressed roller that is located between the pair of pressing rollers to receive pressure applied by the pair of pressing rollers.
In the both-face image forming apparatus, the pair of pressing rollers and the pressed roller nip and convey the sheet. At this time, the sheet is curled by the application of a curling force.
For the both-face image forming apparatus of this invention, the distance to which the pressed roller of the recurling means enters the space between the pair of pressing rollers may be greater than the distance to which the pressed roller of the initial curling means enters the space between the pair of pressing rollers.
In the both-face image forming apparatus, when curling a sheet the recurring means provides a greater curling distance than does the initial curling means. Therefore, a sheet that has been curled by the surface reversing means in the direction in which it can not be smoothly separated from the photosensitive member is then curled in the direction in which it can be easily separated.
For the both-face image forming apparatus, the length of the pair of pressing rollers of the recurling means is shorter than the width of the sheet, and is greater than the length of the pair of pressing rollers of the initial curling means.
The both-face image forming apparatus employs the recurling means to flatten the widthwise ends of the sheet.
For the both-face image forming apparatus of this invention, the initial curling means and the recurling means may each include a roller having a large diameter and a roller having a small diameter for nipping and curling a sheet, and the roller having a large diameter may be more elastic than the roller having a small diameter.
For the both-face image forming apparatus, the roller having a small diameter is made of metal, and the roller having a large diameter is made of sponge.
In the both-face image forming apparatus, the sheet is curled by pushing a part of the roller having a small diameter into the roller having a large diameter.
For the both-face image forming apparatus of this invention, the depth to which the roller of the recurling means having a small diameter bites into the roller having a large diameter is greater than the depth to which the roller of the initial curling means having a small diameter bites into the roller having a large diameter.
For the both-face image forming apparatus of this invention, the force with which the roller having a small diameter of the recurling means presses against the roller having a large diameter is greater than the force with which the roller having a small diameter of the initial curling means presses against the roller having a large diameter.
For the both-face image forming apparatus of the present invention, the roller of the recurling means having a large diameter may be more elastic than the roller of the initial curling means having a large diameter.
For the both-face image forming apparatus of the present invention, the diameter of the roller of the recurling means having a small diameter may be smaller than the diameter of the roller of the initial curing means having a small diameter.
For the both-face image forming apparatus, when the recurling means curls a sheet it provides a greater curling distance than does the initial curling means. Thus, the sheet that has been curled by the surface reversing means in the direction in which it can be ineffectively separated from the photosensitive member is now curled in the direction in which it can be easily separated.
For the both-face image forming apparatus, the initial curling means and the recurling means may each include a pair of pressing rollers separately positioned in the sheet feeding direction, and one pressed roller that is located between the pair of pressing rollers to receive pressure applied by the pressing rollers.
In the both-face image forming apparatus, the pair of pressing rollers and the pressed roller nip and convey the sheet. At this time, the sheet is curled by the application of a curling force.
For the both-face image forming apparatus of this invention, the distance to which the pressed roller of the recurling means enters the space between the pair of pressing rollers may be greater than the distance to which the pressed roller of the initial curling means enters the space between the pair of pressing rollers.
In the both-face image forming apparatus, when curling a sheet the recurling means provides a greater curling distance than does the initial curling means. Therefore, a sheet that has been curled by the surface reversing means in the direction in which it can not be smoothly separated from the photosensitive member is then curled in the direction in which it can be easily separated.
For the both-face image forming apparatus, the curling capability of the recurling means may be greater than the curling capability of the initial curling means.